At the small spatial scales, \(\lambda =2.5m^{-1}\)  and \(\lambda =10m^{-1}\) , for small chirp, the oscillation is not obvious, but we observed peak splitting on momentum spectrum, see Fig. REF (a) and (b). The peak splitting can be explained by ponderomotive effects which have been reported in Ref. [1]}. The magnitude of ponderomotive force is inversely proportional to the size of spatial scale [2]}. Therefore the momentum peaks at spatial scale \(\lambda =2.5m^{-1}\)  are more far away from the center compared with the case of \(\lambda =10m^{-1}\)  in Fig. REF (a) and (b). However, for large chirp, the peaks that split by the ponderomotive force are replaced by strong oscillation as shown in Fig. REF (c) and (d). Because the effective frequency increases obviously with the large chirp and provides greater contribution to frequency of external field. So that corresponding effect of ponderomotive force decreases and strong oscillation occurs on the momentum spectrum. This oscillation can be understood as the interference effects between created particles.
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